It may be desirable for an engine to include a turbocharger and exhaust gas recirculation (EGR) to reduce emissions of NOX, CO, and other gasses and to improve fuel economy. However, low pressure exhaust gas recirculation (LP-EGR) can lead to high temperatures on components in the path of the LP-EGR gasses. For example, the turbocharger compressor inlet can heat up to an undesirable temperature due to the hot LP-EGR gasses. An EGR cooler may reduce the temperature of gasses, but the cooler can also condense water out. This can be problematic in any EGR system, but water droplets formed in a low-pressure EGR circuit can particularly degrade an aluminum compressor wheel of a turbocharger operating at high speed. Similarly, components in a high pressure exhaust gas recirculation (HP-EGR) path can be heated to undesirable temperatures or be exposed to condensate that may degrade the components.
One solution is to maintain a “base” EGR table, and to globally modify the table as needed to maintain temperatures and condensate levels below a threshold level. However, the global solution may be overly conservative resulting in an EGR rate that may be less than desired. The inventors herein have recognized the above issues and have devised an approach to at least partially address them. For example, the temperature and condensate constraints for some engine components may be generally independent and may only be affected in limited and unique areas of the EGR operating region.
In one example, a method for controlling an engine in a vehicle during engine operation is disclosed. The engine includes an intake passage and an EGR system. The method comprises controlling an amount of EGR according to a minimum of a first EGR amount corresponding to a temperature at a first location and a second EGR amount corresponding to condensate formation at a second location. The locations may both be in the LP-EGR system, or one may be in the LP-EGR system and the other in the HP-EGR system, for example. Further, the first location may correspond to a particular component, such as an EGR valve, that has the most severe temperature limitation, whereas the second location may correspond to the location most likely to form condensate, such as at a charge air cooler or at the compressor inlet. For example, at a first operating point, such as at low speed and high load, the condensate constraint at an output of the LP-EGR system upstream of the compressor may be a limit on EGR. At a second operating point, such as at mid-speed and load, the temperature constraint at a valve in the HP-EGR system may limit EGR. In this manner, the EGR rate may be maintained at a desirable level while still operating the engine components below a threshold temperature and reducing condensate formation.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.